Key Process Points for Heating Temperature Control in Glass Tempering Process
In the glass tempering production process, the reasonable selection of heating temperature and effective control of furnace temperature are core links determining product quality, directly affecting the tempering strength, flatness and yield rate of glass. The formation principle of temperedglass is to heat the glass to a softened state at high temperature, then form surface compressive stress and internal tensile stress through rapid and uniform cooling, thereby significantly improving the mechanical properties and safety performance of glass. The foundation of this series of physical changes lies in precise temperature control and scientific process parameter setting. This article will elaborate on key points such as heating temperature selection, furnace temperature control, heating time setting, glass arrangement specifications, cooling process requirements and glass movement control in combination with production practice.
I. Core Logic of Reasonable Selection of Heating Temperature and Effective Control of Furnace Temperature
In glass tempering production, the load condition of the electric furnace is the core basis for determining the heating temperature. However, it should be clarified that the electric furnace load mentioned here does not refer to the plane area occupied by glass in the electric furnace, but specifically refers to the dynamic balance relationship betweenglass thickness, heating temperature and heating time. This relationship runs through the entire tempering heating process and is the fundamental principle for formulating heating process parameters. Different thicknesses of glass have significant differences in heat demand: thin glass has a fast heating rate and small heat capacity, while thick glass is the opposite. Ignoring this difference and setting the temperature blindly can easily lead to problems such as uneven heating, overheating or underheating of glass.
From the perspective of mainstream production equipment in the industry, the heating section of tempered electric furnaces used by most manufacturers adopts a zoned heating design, which can be divided into multiple independent small heating zones. The core advantage of this design is that it can realize targeted temperature regulation and ensure the uniformity of the temperature field in the furnace. Under normal production conditions, there is always glass in the heating area of the heating element at the midpoint of the electric furnace that is absorbing heat, and the continuous transportation of glass is maintained in the entire working area of the electric furnace, forming a regional balance between heating and heat absorption. This regional balance directly determines the local heating effect. When the heat consumption rate in a certain area exceeds the heat supply rate of the heating element, the temperature in that area will drop significantly, which is the formation of overload phenomenon.
It should be emphasized that the success of glass tempering depends on the heating quality of the low-temperature area of the glass sheet. As a poor conductor of heat, if local temperature drop occurs in the furnace, it will lead to excessive temperature difference in various parts of the glass sheet. In the subsequent cooling stage, the shrinkage rate of different areas is inconsistent, generating huge internal stress. When this internal stress exceeds the bearing capacity of the glass itself, it will cause glass breakage and production loss. Therefore, effectively avoiding the overload phenomenon and maintaining the stable temperature of each area in the furnace are the core objectives of heating temperature control.
To realize the effective control of furnace temperature, in addition to accurately setting the heating temperature according to the load condition, it is also necessary to equip a complete temperature monitoring and feedback regulation system. By arranging temperature sensors in different areas of the furnace, real-time temperature data can be collected and transmitted to the control system. When it is detected that the temperature in a certain area deviates from the set value, the system can automatically adjust the power of the heating element in that area to make up for the heat loss in time. At the same time, operators need to regularly inspect and calibrate the heating elements and temperature sensors to ensure that the equipment is in good working condition and avoid temperature control failure caused by equipment faults. In addition, the sealing performance of the furnace body also affects temperature stability. Problems such as poor sealing of the furnace door and damage to the thermal insulation layer of the furnace body will cause heat loss and destroy the balance of the temperature field in the furnace. Therefore, daily maintenance of the furnace body should be strengthened to ensure the sealing and thermal insulation effect.
II. Scientific Setting of Heating Time to Ensure Sufficiency and Uniformity of Heating
On the basis of determining the heating temperature, the reasonable setting of heating time is also crucial. The heating power of the tempering furnace is basically fixed when the equipment leaves the factory, so the heating time becomes a key parameter for adjusting the heat absorption of glass. If the heating time is too short, the glass cannot reach a fully softened state, and a uniform stress layer cannot be formed after cooling, resulting in insufficient tempering strength. If the heating time is too long, the glass is prone to over-softening, leading to surface deformation, edge bending, and even defects such as bubbles and stones, which also affect product quality.
Combined with industry production experience, the setting of heating time usually takes glass thickness as the core basis, forming a relatively mature reference standard: for glass of conventional thickness, the heating time is about 35~40 seconds per millimeter of thickness. For example, when producing tempered glass with a thickness of 6mm, the heating time can be set according to the standard of 6×38 seconds = 228 seconds (38 seconds is the intermediate reference value in the range of 35~40 seconds, and can be fine-tuned according to factors such as glass type and ambient temperature in actual production). For thickglass with a larger thickness of 12~19mm, due to its lower heat conduction efficiency, a longer heating time is required to ensure sufficient internal heating. Therefore, the basic calculation method of heating time is adjusted to 40~45 seconds per 1mm thickness.
It should be noted that the above heating time standard is only a basic reference, and flexible adjustment should be made by comprehensively considering various factors in actual production. For example, different types ofglass have differences in physical properties such as specific heat capacity and softening temperature, so the heating time of ordinary float glass and Low-E coated glass needs to be different. Changes in ambient temperature will also affect heating efficiency. In low-temperature environments in winter, the initial temperature of glass is low, and the heating time needs to be appropriately extended. In addition, the placement density of glass in the electric furnace and the air flow state in the furnace will also affect the heating time. Therefore, operators need to continuously accumulate experience in the production process and dynamically optimize the heating time according to the actual production situation to ensure the sufficiency and uniformity of glass heating.
III. Optimizing Glass Placement Arrangement to Ensure Uniformity of Furnace Load
To realize the uniform heating of glass, in addition to precise control of temperature and time, the arrangement method of glass on the sheet feeding table also plays an important role. The core goal of reasonable placement arrangement is to ensure the uniformity of vertical and horizontal loads in the electric furnace, avoid local glass being too dense or too sparse, thereby maintaining the stability of the temperature field in the furnace and improving the overall heating effect.
Specifically, the standard requirements for placement arrangement mainly include the following two aspects:
Uniform placement layout of glass in a single furnace: When placing glass, it is necessary to reasonably allocate the placement position of each piece of glass according to the size of the electric furnace and the division of heating zones, ensure that the distance between adjacent glass is consistent, avoid placing too much glass in a certain heating zone, leading to excessive load and insufficient heat supply in that zone. At the same time, it is also necessary to avoid glass being placed too scattered, resulting in heat waste and local excessive temperature. When producing glass of different sizes and thicknesses in mixed loading, more attention should be paid to the rationality of the layout, and glass with similar thickness and size should be placed centrally to facilitate precise control of heating parameters.
Uniform interval time between each furnace of glass: In the continuous production process, the time interval between the outgoing of glass from the previous furnace and the incoming of glass to the next furnace needs to be kept stable. If the interval time is too long, the temperature in the furnace will fluctuate significantly, and the subsequent glass entering the furnace will take a longer time to reach the set temperature. If the interval time is too short, the heat taken away by the glass from the previous furnace has not been supplemented, and the glass from the next furnace enters the furnace, which will cause a sudden drop in the temperature in the furnace and trigger an overload phenomenon. Therefore, operators need to set a reasonable inter-furnace interval time according to factors such as the heating power of the electric furnace and the heating demand ofglass, and strictly implement it through automatic control systems or manual operations to ensure the stability of the production rhythm.
Through the above standard placement arrangement, the uniformity of the furnace load can be effectively guaranteed, providing basic conditions for the uniform heating of glass.
IV. Precisely Controlling the Cooling Process to Ensure Tempering Quality
After heating, the glass enters the cooling stage. The cooling rate and cooling uniformity directly determine the tempering effect of the glass. According to the formation principle of temperedglass, the glass in a softened state needs to be cooled as quickly as possible to form a uniform compressive stress layer on the surface. However, the cooling rate is not as fast as possible. It needs to match the thickness, type and other properties of the glass. At the same time, it is necessary to ensure the balanced cooling of the front and back sides of the glass to avoid internal stress caused by uneven cooling leading toglass breakage.
The core influencing factors of cooling rate include glass thickness and glass physical properties. Generally speaking, the cooling rate of thin glass can be appropriately increased, while the cooling rate of thick glass needs to be controlled to avoid cracks caused by excessive temperature difference between inside and outside. For example, the thickness of 5mm glass is relatively thin, and the heat conduction rate is relatively fast. The required cooling capacity is more than twice that of 6mm glass. This is because thin glass loses heat quickly during the cooling process and needs stronger cooling capacity to achieve rapid and uniform cooling. However, thickglass loses heat slowly. If the cooling capacity is too strong, it will cause the surface to cool and shrink rapidly, and the internal heat cannot be dissipated in time, forming a huge temperature gradient and internal stress, leading to breakage.
In the selection of cooling medium, the ideal cooling medium for the cooling stage in the tempering process is dry cold air. Dry cold air can avoid the condensation of moisture on the surface of glass, prevent defects such as watermarks and fog spots onglass, and at the same time, the specific heat capacity of cold air is stable, and the cooling effect is uniform and controllable. To ensure the cooling effect, the air volume and wind speed of the cooling system need to be precisely adjusted according to the glass thickness to ensure that the cooling capacity per unit area meets the set standard. In addition, the design of the cooling air grid also needs to be scientific and reasonable. The air outlets of the air grid should be evenly distributed to ensure that the front and back sides of the glass can obtain the same cooling air volume and wind speed, realizing balanced cooling.
V. Controlling Glass Movement State to Avoid Surface Defects and Breakage Risks
In the entire tempering process, the movement state of glass has a direct impact on product quality. It is required that the glass maintains continuous and stable movement during the production process, and there should be no scratches or marks left by deformation on the glass surface. This movement mainly includes the following two stages:
Hot swing movement in the heating furnace: Its core purpose is to enable each part of the glass surface to absorb heat uniformly. Due to the possible slight temperature difference in different areas of the electric furnace, the glass can make different parts of the surface alternately in different heating areas through slow reciprocating swing, thereby making up for the slight unevenness of the temperature field and ensuring the uniform heating of the entire glass. The speed and amplitude of the hot swing movement need to be strictly controlled. Excessively fast speed may cause the glass to collide with the furnace components, resulting in surface scratches. Excessively slow speed cannot achieve the effect of uniform heating. Excessively large amplitude may cause bending deformation of the glass edge, and excessively small amplitude makes the effect of uniform heating not obvious.
Cold swing movement in the air cooling section: It is mainly to ensure the uniform cooling of glass, and then make the broken pieces of glass uniform after breaking. During the cooling process, the glass can make each part of the surface evenly contact the cooling air flow through reciprocating swing, avoiding local excessive or slow cooling. Uniform cold swing movement can ensure the uniform distribution of compressive stress on the glass surface, which not only can improve the tempering strength of glass, but also ensure that when the glass breaks due to impact, the broken pieces present uniform small particles, meeting the standard requirements of safety glass.
In addition to the control of the movement state, the quality of the original glass also has an important impact on the tempering effect. The original glass should not have defects such as scratches, bubbles, stones and cracks. These defects will become stress concentration points. During the heating and cooling process, the stress at the defect location will increase sharply, eventually causing glass breakage. Therefore, it is necessary to strictly inspect the original glass before production, remove the glass with defects, and ensure the quality of tempered glass products from the source. At the same time, during the handling and placement of glass, protective measures should be taken to avoid scratches or collision damage on the glass surface.
VI. Conclusion
In summary, links such as heating temperature selection, furnace temperature control, heating time setting, glass arrangement, cooling process and glass movement control in the glass tempering process are interrelated and mutually influential, jointly determining the product quality of tempered glass.
In actual production, operators need to deeply understand the core logic of each process point, accurately set the heating temperature and heating time based on basic parameters such as glass thickness and type, optimize theglass placement arrangement, strictly control the cooling rate and uniformity, standardize the control of glass movement state, and strengthen the inspection of original sheets and equipment maintenance.
Only through comprehensive and refined process control can the yield rate and quality stability of tempered glass be effectively improved, meeting the performance requirements of tempered glass in different application scenarios, and promoting the high-quality development of the glass tempering production industry.
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